Icahn, who has a 0.56 percent stake in Cigna, on Tuesday made public his opposition to the health insurer’s $52 billion purchase of Express Scripts Holding Co, citing regulatory hurdles and the growing threat from Amazon.

“Everyone wants drug prices lower — it is a bipartisan issue. I have little doubt that legislation will ultimately be passed to do away with rebates,” Icahn said in a letter on Thursday.

PBMs have come under increasing scrutiny in the drug pricing debate, with the Trump administration proposing a rule that would scale back protections currently in place that allow rebates between drug manufacturers, insurers and PBMs.

Rebates have been cited as one of the reasons for escalating drug prices, but PBMs such as CVS Health have downplayed the importance of rebates to their business.

The 82-year old billionaire has also highlighted the rising threat of Amazon, which in June bought small online pharmacy PillPack, putting the world’s biggest online retailer in direct competition with drugstore chains, drug distributors and PBMs.

Last week, Cigna said it was confident it would secure shareholder approval for the deal. The vote is scheduled for Aug. 24.

The insurer also said on Tuesday that it strongly disagrees with Icahn.

“Mr. Icahn has made clear through his recent and limited entry into Cigna stock and ‘substantial’ short position in Express Scripts that his motives are not aligned with Cigna’s shareholders and he has no interest in creating value for shareholders (other than himself).” (Reporting by Ankur Banerjee in Bengaluru; Editing by Saumyadeb Chakrabarty and Shounak Dasgupta)